Developments in optical data transmission systems involve the use of integrated opto-electronic circuits for the connection between optical and electronic components. The problem of integration of the photodetector and pre-amplifier arises at the receiver side. Good compatibility of the semiconductor layers and manufacturing techniques required for the various components is required. A monolithically integrated, planar module that contains a photodiode and a field effect transistor (FET) on a semiconductor substrate and that takes the different requirements of doping concentration and layer thickness into consideration is recited in H. Albrecht, Ch. Lauterbach, "Monolithically Integrated InGaAs/InP:Fe Photodiode-Junction Field-Effect Transistor Combination", Siemens Forschungs-und Entwicklungsberichte 17, 195-198 (1988), incorporated herein by reference.
For a high external quantum efficiency, an optimized photodiode requires an adequately thick absorption layer of n-InGaAs having low residual doping. This nominally undoped InGaAs layer is a prerequisite for a low capacitance as well as a low dark current and, consequently, good noise properties. A highly doped n-conductive semiconductor layer (for example, InGaAs or InAlAs or InP) on the InP:Fe substrate reduces the series resistance and the transient time effects, i.e. the carrier transient time of the photodiode.
For a high transconductance, the FET requires a highly doped n.sup.+ --InGaAs layer that must be correspondingly thin for a good cut-off behavior of the FET. The cover layer of n-InP or N-InAlAs forms a hetero barrier vis-a-vis the InGaAs and enables the realization of a blocking metal-to-semiconductor junction as a gate, or enables the manufacture of a barrier layer gate together with a p-diffusion or p-implantation. This layer also has the advantage that, due to the higher band spacing of InP or InAlAs compared to InGaAs, a passivation of the pn-junction at the component surface lengthens the long-term stability and reduces leakage currents. This advantage results both for the photodiode as well as for the FET.